RFC on writel and writel_relaxed

[Nicholas Piggin]

On Thu, 29 Mar 2018 08:31:32 +1100
Benjamin Herrenschmidt <benh at kernel.crashing.org> wrote:

> On Thu, 2018-03-29 at 02:23 +1000, Nicholas Piggin wrote:
> > On Wed, 28 Mar 2018 11:55:09 -0400 (EDT)
> > David Miller <davem at davemloft.net> wrote:
> >   
> > > From: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> > > Date: Thu, 29 Mar 2018 02:13:16 +1100
> > >   
> > > > Let's fix all archs, it's way easier than fixing all drivers. Half of
> > > > the archs are unused or dead anyway.    
> > > 
> > > Agreed.  
> > 
> > While we're making decrees here, can we do something about mmiowb?
> > The semantics are basically indecipherable.  
> 
> I was going to tackle that next :-)
> 
> >   This is a variation on the mandatory write barrier that causes writes to weakly
> >   ordered I/O regions to be partially ordered.  Its effects may go beyond the
> >   CPU->Hardware interface and actually affect the hardware at some level.
> > 
> > How can a driver writer possibly get that right?
> > 
> > IIRC it was added for some big ia64 system that was really expensive
> > to implement the proper wmb() semantics on. So wmb() semantics were
> > quietly downgraded, then the subsequently broken drivers they cared
> > about were fixed by adding the stronger mmiowb().
> > 
> > What should have happened was wmb and writel remained correct, sane, and
> > expensive, and they add an mmio_wmb() to order MMIO stores made by the
> > writel_relaxed accessors, then use that to speed up the few drivers they
> > care about.
> > 
> > Now that ia64 doesn't matter too much, can we deprecate mmiowb and just
> > make wmb ordering talk about stores to the device, not to some
> > intermediate stage of the interconnect where it can be subsequently
> > reordered wrt the device? Drivers can be converted back to using wmb
> > or writel gradually.  
> 
> I was under the impression that mmiowb was specifically about ordering
> writel's with a subsequent spin_unlock, without it, MMIOs from
> different CPUs (within the same lock) would still arrive OO.

Yes more or less, and I think that until mmiowb was introduced, wmb
or writel was sufficient for this.

Thanks,
Nick